Magnetic memories, e.g., magnetic random access memories (MRAMs), have drawn increasing attention in nonvolatile memory applications due to their properties of high read-write speed, excellent endurance, and low power consumption.
A typical MRAM system contains two ferromagnetic (FM) layers separated by an ultrathin insulator, i.e., a magnetic tunnel junction. The magnetization of one FM layer, a pinned or reference layer, is pinned by an anti-ferromagnetic (AFM) layer, whereas the magnetization of the other FM layer, i.e., the free layer, is relatively free to rotate. The relative direction of the two FM layers is used to store information.
Traditionally, magnetization switching of the free layer is accomplished by applying an external field of appropriate direction and strength. This process has a drawback of inapplicability for downscaling and low-power operation.
Spin transfer torque (STT) has been used in MRAMs for improving their magnetization capability. Yet, STT-MRAM has drawbacks such as requiring high current density and same current for both write and read. The latter makes it difficult to optimize the cell design for both write and read operations.
Recently, extensive efforts have been devoted to the study of spin-orbit torque (SOT), which is based on spin-orbit coupling, in bilayer magnetic systems, e.g., ferromagnetic metal/heavy metal (FM/HM) bilayers. Despite the capability of switching the magnetization of a thin FM layer, use of SOT in FM/HM bilayers faces a significant challenge for downscaling when applied in magnetic memories due to the poor thermal stability of the thin layer. Another drawback of this application resides in the requirement of high current density and an assistant field.
There is a need to develop a new magnetic system that is capable of switching magnetization without the above-described drawbacks.